This invention relates generally to active noise control and more particularly to active suppression of acoustic radiation from a complex vibrating surface.
Traditional methods of passive noise control include placing the noise source on shock mounts or in an enclosure, redesigning the moving parts of a noisy machine, constructing physical barriers between offending noise sources and human listeners, and using sound-absorbing materials to reduce reverberations, e.g. in large rooms. These methods are most effective at frequencies above about 500 Hz, where the wavelengths are relatively short. Low-frequency noise is more difficult to control by passive means because its wavelengths generally exceed the dimensions of practical barriers and other acoustical treatments.
Many noise problems that cannot be solved by passive methods are candidates for active noise control, which is based on the principle of destructive interference. In a designated "quiet zone," the undesired noise is mixed with electronically generated "antinoise," which has the same amplitude as the original noise but the opposite phase. Thus, the two noise fields tend to cancel each other over a specified frequency band. The required accuracy for generating the correct antinoise is inversely related to frequency, so that, in practice, active noise control is especially useful for attenuating low-frequency noise. It follows from this discussion that passive and active methods of noise control are complimentary.
The prior art of active noise control comprises two classes of methods and apparatus: single-channel and multichannel control systems.
Single-channel controllers have an input called the reference signal, which represents the undesired noise. The reference signal may be a predetermined waveform (for periodic noise) or derived from an input sensor such as a microphone (for random noise). The controller has a single output that is fed to an output transducer (such as a loudspeaker) to produce the required antinoise. Controllers of this kind usually implement algorithms that provide a model of the acoustical plant, which may include a feedback path from the output transducer to the input sensor. There is a second input called the error signal, which describes the performance achieved in the quiet zone. The error signal is used to adapt the model in such a manner as to minimize the residual noise.
Depending on the application, the reference signal may be a tone or broadband noise. In U.S. Pat. No. 5,010,576, Hill provides an example of a single-channel controller that uses a tone as the reference signal. An accelerometer attached to a fan motor tracks the blade-passage frequency of the fan noise. In this example, there is no acoustic feedback path because an accelerometer does not respond to airborne noise. A contrasting example of a controller with a broadband reference signal is given by Allie et al. in U.S. Pat. No. 4,736,431. Here a microphone placed in a ventilation duct monitors the entire spectrum of the fan noise to be canceled. A special feature of this controller is that it is calibrated automatically; the algorithm converges even if the fan noise contains tones. Regardless of the type of reference signal, the identifying feature of a single-channel system is that there is a single forward signal path through the controller.
Multichannel control systems are needed when the sound is not limited to plane waves traveling in a duct but is propagating in all directions. Multichannel controllers use many input signals that describe the spatial distribution of one or more noise sources, and many output signals that specify the antinoise required in different spatial regions of the quiet zone. The inputs are connected to the outputs by means of forward filters, which are usually implemented by finite-impulse-response digital filters. If the acoustical plant includes feedback from output transducers to input sensors, the controller must also contain neutralization filters to correct for this feedback. Properly adjusted neutralization filters provide system stability and increase system performance.
Depending on the application, multichannel controllers may also use error signals from the quiet zone to adapt the forward and neutralization filters to changes in the acoustical plant. An example of a multichannel system with many inputs and outputs is provided by Martinez et al. in U.S. Pat. No. 5,224,168. Regardless of the particular configuration, the identifying feature of a multichannel controller is that at least one input is connected to two or more outputs by means of separate forward filters. In a fully interconnected controller, each input is connected to all outputs, and the number of forward filters is equal to the number of inputs multiplied by the number of outputs.